fix(channels): center table, kill popover top-left flash, exact "+N" count (R8 S1 UAT)

Second UAT pass on the tag cell + popover platform:

- Popover top-left flash (real root cause): the popIn keyframe animates `transform`,
  which overrode Floating UI's positioning `transform: translate(x,y)` for the animation's
  duration, parking the popover at (0,0). Move the animation to an INNER wrapper so the
  positioned element's transform stays clean (verified: menu is at the correct spot from
  frame 0). The isPositioned visibility gate is kept as a belt-and-suspenders.
- Table shifted right / not centered: my previous fixed-15rem tags column + nowrap Sync
  pushed the table past the max-w-[96rem] wrapper so it overflowed. Trim to 12rem tags and
  shorten the long "full history queued/coming" sync labels to "queued"/"coming" so the
  table fits the wrapper (1594 <= 1628) and centers again.
- "+N" count wrong/missing while adding tags: replace the iterative shrink (which lagged a
  render and could clip the pill's digits) with a deterministic measure — a hidden measurer
  lays out all chips + a sample pill, and we compute the exact fit in one pass. The count is
  always correct now (verified across varied rows), and a bare "+" with no number is
  structurally impossible.

frontend/src/i18n/locales/en/channels.json

@@ -107,8 +107,8 @@
     "backfillThisHint": "Fetch this channel's full back-catalog (older videos + complete search). Click again to cancel the request.",
     "resetBackfill": "Reset & re-fetch this channel",
     "resetHint": "Admin: re-fetch this channel from scratch (recent + full back-catalog), regardless of its current sync state.",
-    "fullHistoryQueued": "full history queued",
-    "fullHistoryComing": "full history coming",
+    "fullHistoryQueued": "queued",
+    "fullHistoryComing": "coming",
     "queuedRequestedHint": "Full history requested — this channel's whole back-catalog will backfill as the shared quota allows. Click to cancel your request.",
     "queuedByOtherHint": "Another subscriber already requested this channel's full history, so its whole back-catalog is on its way to everyone — nothing to do here.",
     "hiddenHint": "Hidden — this channel's videos are kept out of your feed. Click to show them again.",

frontend/src/i18n/locales/hu/channels.json

@@ -107,8 +107,8 @@
     "backfillThisHint": "A csatorna teljes archívumának letöltése (régebbi videók + teljes keresés). Újra kattintva visszavonod a kérést.",
     "resetBackfill": "Csatorna resetelése és újraletöltése",
     "resetHint": "Admin: a csatorna teljes újraletöltése a nulláról (legutóbbi + teljes archívum), a jelenlegi szinkron-állapottól függetlenül.",
-    "fullHistoryQueued": "teljes előzmény sorban",
-    "fullHistoryComing": "teljes előzmény úton",
+    "fullHistoryQueued": "sorban",
+    "fullHistoryComing": "úton",
     "queuedRequestedHint": "Teljes előzmény kérve — a csatorna teljes archívuma letöltődik, ahogy a megosztott kvóta engedi. Kattints a kérés visszavonásához.",
     "queuedByOtherHint": "Egy másik feliratkozó már kérte ennek a csatornának a teljes előzményét, így a teljes archívuma már úton van mindenkihez — itt nincs teendőd.",
     "hiddenHint": "Elrejtve — a csatorna videói nem jelennek meg a hírfolyamodban. Kattints, hogy újra láthatóak legyenek.",